The invention relates to a semiconductor device having a semiconductor body with a substantially flat surface, comprising a substrate region of a first conductivity type, a first epitaxial layer of the second, opposite conductivity type on said substrate region and a second epitaxial layer of the first conductivity type on said first epitaxial layer, which device comprises a semiconductor resistor having a strip-shaped resistance zone of the second conductivity type extending from the surface into the first epitaxial layer and having a higher doping concentration than the first epitaxial layer, which resistance zone is situated between two parallel grooves extending from the surface into the substrate region, and a surface-adjoining strip-shaped surface zone of the first conductivity type which is provided in said resistance zone and which within the semiconductor body is surrounded entirely by the resistance zone.
A semiconductor device having a semiconductor resistor of the kind described is known from published French Patent Application No. 2,335,957 of R.T.C. Compelec, priority date Dec. 17, 1975. Said application describes an integrated Darlington circuit in which the said semiconductor resistance connects the base zones of the input transistor and the output transistor. This resistor is provided in a zig-zag shape; it is to be noted that the said parallel grooves are to be considered as parallel when their shortest distance is substantially equal everywhere. However, they need not extend as straight lines.
It has proved difficult to give this resistor an accurately determined and reproducible value, which, however is strictly necessary, notably in such integrated circuits as Darlington circuits.
The cause of this difficulty resides in the fact that the resistor on either side of the surface zone of the first conductivity type comprises a part of the resistance zone adjoining the wall of the groove and its cross-section is hard to determine with the required precision. In fact, this depends on the precision with which the grooves can be etched. This in contrast with the dopings, the depths and the extent of the various semiconductor regions which further determine the resistance and which can be controlled with considerable precision.
For example, in a practical case the groove width on the top side is 80 .mu.m to 100 .mu.m, said width showing variations of 5 .mu.m to 20 .mu.m as a result of deviations in the etching process, the tolerance of the etching mask (at least 5 .mu.m) being also of importance. The parts of the resistance zone extending on either side of the surface zone and which in the practical case described here each have a width of 10 .mu.m to 15 .mu.m, may therefore show very large relative differences. In an extreme case, the groove may even touch the said surface zone so that at least one of the lateral parts of the resistance zone disappears. The said deviations in the cross-section of the lateral parts of the resistance zone may lead to unacceptable deviations in the value of the semiconductor resistance.